This invention relates to the field of rotating electrical machines with asynchronous technology.
More particularly, this invention relates to rotors used in such asynchronous rotating electrical machines.
An asynchronous rotating electrical machine, also called an “induction machine”, is an alternating-current electrical machine principally comprising a stator, a rotor without a physical connection with the stator, and a torque transmission shaft integral with the rotor. Said rotor comprises a winding consisting of short-circuited conductors and traversed by currents induced by a magnetic field created by the currents of the stator, unlike a synchronous machine wherein the magnetic field of the rotor is created by permanent magnets or coils supplied by direct current.
The rotor of an asynchronous electrical machine may, for example, be of the “squirrel-cage” type, comprising a plurality of layers of axially stacked ferromagnetic metal sheets forming a magnetic ring and a squirrel cage. For example, a magnetic ring of 1,000 mm axial length comprises two thousand layers of axially stacked ferromagnetic metal sheets, each of 0.5 mm thickness, or one thousand five hundred and thirty-eight layers of axially stacked ferromagnetic metal sheets, each of 0.65 mm thickness.
In a manner known in itself, the squirrel cage comprises a plurality of conductive elements regularly distributed over the periphery of the magnetic ring, each having two opposite ends extending axially beyond the magnetic ring. The conductive elements are made of an electrical material, such as copper, a copper alloy or aluminum. The squirrel cage further comprises two rings or short-circuit crowns, arranged axially on either side of the magnetic ring and intended to connect the ends of the conductive elements electrically. Each conductive element is in the form of a bar extending axially on either side of the magnetic ring and inserted axially in a notch cut close to the periphery of the magnetic ring. The section of the conductive bars may be circular, ovoid, square or rectangular in shape.
The ferromagnetic sheets are intended to guide the magnetic field lines, while the conductive elements are intended to accommodate the induced currents. The magnetic ring and the squirrel cage thus form the active part of the rotor.
When the external diameter of the rotor exceeds 1,250 mm, it is difficult to produce ferromagnetic sheets to be cut into magnetic sheet rolls with a width greater than 1,250 mm. Thus, there are magnetic rings comprising a plurality of layers of ferromagnetic metal sheets, wherein each sheet metal layer comprises ferromagnetic sheet segments, for example 18 in number, each forming an angular portion of 20°, stacked in the circumferential direction. For reasons of mechanical rigidity, the angular portions of each layer of sheets are offset angularly in order to avoid the alignment of the joining planes between the segments, which would weaken the magnetic ring. Thus, the axial covering of the segments from one layer of sheets to the other makes it possible to ensure the rigidity of the magnetic ring.
However, in the event of failure of a part of the rotor, it is necessary to disassemble the rotor as a whole, which involves a considerable maintenance cost.
In addition, it is necessary to assemble such a rotor directly on the operating site of the torque transmission shaft, which is generally difficult to do and expensive.
There is therefore a need to provide an asynchronous rotor for an asynchronous electrical machine capable of being manufactured on a site other than the operating site of the torque transmission shaft. There is also a need to reduce the time and cost of maintaining such an asynchronous rotor.